Wire wound resistor and method of making the same



B. SOLOW May 30, 1961 WIRE WOUND RESISTOR AND METHOD OF MAKING THE SAME Filed May 20, 1959 9 l 24 i I8 '6 24a num- XXX INVENTOR. BENJAMIN SOLOW ATTORNEY United States Patent WIRE WOUND RESISTOR AND METHOD OF MAKING THE SAME Benjamin Solow, North Hollywood, Calif., assignor to International Resistance Company, Philadelphia, Pa.

Filed May 20, 1959, Ser. No. 814,437

3 Claims. (Cl. 29-155.

The present invention relates to a wire wound resistor and the method of making the same, and more particularly to an encapsulated wire wound resistor and the method of making the same.

In general, a wire wound resistor comprises a bobbin or core of an electrical insulating material having a wire of an electrical resistance material wound thereon. The ends of the resistance wire are connected to terminals which are secured to the bobbin and extend either longitudinally or radially from the bobbin. An encapsulated wire wound resistor includes a jacket or casing of an electrical insulating plastic which is molded or cast around the bobbin and the resistance wire. The resistance value of such a wire wound resistor is determined by the length of the resistance wire wound on the bobbin, and the resistance value per unit length of the resistance wire.

In making a precision wire wound resistor, i.e. a resistor having a resistance value which is as close as possible if not exactly the desired resistance value, it is necessary to accurately measure the amount of resistance wire wound on the bobbin. The most accurate method heretofore used for measuring the amount of resistance wire wound on the bobbin is to continuously measure the resistance value of the wire as it is wound on the bobbin. When the desired resistance value is reached, the wire is broken and the ends of the wire are secured to the terminals of the resistor. However, to permit the ends of the wire to be secured to the terminals, extra Wire must be provided at each end of the wire on the bobbin. The amount of extra wire required depends on the skill of the operator who is making the resistor. Therefore, even with a skilled operator, the amount of extra wire used to secure the resistance wire to the terminals will vary from unit to unit so that the resistance values of the resistor units will also vary to some degree.

In making an encapsulated wire wound resistor, the molding or casting of the plastic material around the bobbin and the resistance wire causes a change in the resistance value of the wire, which is known as the mold change. It has been found that the mold change generally causes a decrease in the resistance value of the resistor. In order to compensate for the mold change in resistance value, it has been the practice to make the resistance value of the wire which is wound on the bobbin slightly higher than the final resistance value desired so that upon encapsulating the resistor, the mold change will bring the resistance value of the resistor to the desired value. However, the mold change is not always uniform. Therefore, in the mass production of encapsulated wire Wound resistors, this method of compensating for the mold change is not satisfactory for making precision resistors whose values are all substantially identical.

It is an object of the present invention to provide a novel method for making wire wound resistors.

It is another object of the present invention to provide a method for making precision wire wound resistors.

It is still another object of the present invention to provide a method for making precision encapsulated wire wound resistors.

It is a further object of the present invention to provide a novel wire wound resistor.

It is a still further object of the present invention to provide a novel encapsulated wire wound resistor.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

Figure l is a sectional view of the encapsulated wire wound resistor of the present invention illustrating one stage in the method of making the resistor.

Figure 2 is an elevational view of the encapsulated wire wound resistor of the present invention illustrating a further stage in the method of making the resistor.

Figure 3 is an elevational view of the completed en capsulated wire wound resistor of the present invention.

Referring initially to Figure l, the wire wound resistor of the present invention is generally designated as 10.

Resistor it) comprises a cylindrical bobbin 12 of an electrical insulating material, such as a ceramic or a plastic. Bobbin 12 has a central hole 14 extending longitudinally therethrough, and a pair of longitudinally spaced, annular grooves 16 and 18 in its outer periphery. The groove 16 in the bobbin 12 is deeper than the groove 18 so that the bottom surface of the groove 16 is of a smaller diameter than the bottom surface of the groove 18. Terminal wires 20 and 22 of electrical conducting metal are secured in opposite ends of the hole 14 in the bobbin 12. The inner ends of the terminal wires 20 and 22 are spaced from each other, and the terminal wires 20 and 22 project longitudinally beyond the ends of the bobbin 12.

A continuous length of an electrical resistance wire, generally designated as 24, is helically wound around the bobbin 12 within the grooves 16 and 18. A major portion of the length of the resistance wire 24, such major portion being designated as 24a, is wound in a plurality of substantially concentric layers within the groove 16 in the bobbin 12. A minor portion of the length of the resistance wire 24, such minor portion being designated as 24b, is wound in a single layer in the groove 18 in the bobbin 12. In the embodiment of the resistor 10 of the present invention shown in Figure 1, one end 26 of the resistance wire 24 extends from the groove 16 to the adjacent end of the bobbin 12 through an angled hole 30 in the bobbin 12. The end 26 of the resistance wire 24 is secured to the terminal wire 20, such as by soldering or welding. The other end 28 of the resistance wire 24 extends from the groove 18 to the other end of the bobbin 12 through an angled hole 32 in the bobbin 12, and is secured to the terminal wire 22.

In accordance with the method of the present invention, the resistance wire 24 is wound on the bobbin 12 by first threading the end 26 of the resistance wire 24 through the hole 30, and securing the end 26 of the wire 24 to the terminal wire 20 which has been previously secured in the hole 14 in the bobbin 12. The resistance wire 24 is then helically wound around the bobbin 12 within the groove 16 in a plurality of substantially concentric layers to provide the major portion 24a of the length of the resistance wire 24. As the resistance wire 24 is wound in the groove 16 in the bobbin 12, the resistance value of the wire 24 being wound on the bobbin 12 is continuously measured. When a sufiicient amount of the resistance wire 24 has been wound in the groove 16 so that the resistance value of the wire 24 within the groove 16 is slightly less than the desired total resistance value of the wire 24 to be wound on the bobbin 12, the winding is stopped. The resistance wire 24 is then transferrcd to the groove 18 in the bobbin 12 by passing the wire 24 through a notch 34 in the flange 36 between the groove 16 and the groove 18. The resistance Wire 24 is then wound around the bobbin 12 within the groove 13 in a single layer while still measuring the total resistance value of the wire 24 wound on the bobbin 12. The minor portion 24b of the length of the resistance wire 24 is obtained when the desired total resistance value of the wire 24 wound on the bobbin 12 is reached. The wire 24 is then broken away from the remaining. resistance wire on the supply spool from which the resistance wire 24 is obtained,. and the end 28 of the resistance wire 24 is threaded through the hole 32 and secured to the terminal wire 22. Since the minor portion 24b of the resistance wire 24 is only a very small portion of the total length of the resistance wire 24, the minor portion 24!) of the wire 24 provides only a small percentage of the total resistance value of the resistor 10.

When making an encapsulated wire wound resistor of the present invention, a protec'ive jacket or casing 38 of an electrical insulating material, such as a plastic, is then provided around the portion of the bobbin 12 which has the groove 16 therein to cover the major portion 24a of the resistance wire 24. The protective jacket 33 also extends across the end of the bobbin 12 and sealingly engages a portion of the terminal wire 28 which is adjacent the end of the bobbin 12. The protective jacket 38 may be applied by placing the bobbin and resistance wire assembly in a suitable mold, and either molding or casting the protective jacket 38 therearound. The application of the protective jacket 38 around the major portion 24a of the resistance wire 24 causes a mold change in the resistance value in the resistance Wire 24, which normally decreases the resistance value of the resistance wire 24. For reasons which will appear below, the amount of the resistance wire 24 which is wound on the bobbin 12 is such that after the mold change caused by the application of the protective jacket 38-, the resistance value of the resistance wire 24 is slightly less than the final resistance value desired.

Referring to Figure 2, there is shown the next operation in the method of the present invention, which is the operation of adjusting the resistance value of the resistance wire 24 to the final desired value. The adjusting operation comprises grinding the outer surface of the turns of the minor portion 24b of the resistance wire 24. By grinding the outer surface of the turns of the minor portion 24b of the resistance wire 24, the crosssectional area of each of the turns is reduced so that the resistance value of the resistance wire 24 is increased. Since, as stated above, the resistance value of the resistance wire 24 after the protective jacket 38 is applied to the core 12 is slightly below the final desired resistance value, the adjusting operation is carried out to increase the resistance value of the resistance wire 24 until the final desired resistance value is obtained.

More particularly, the adjusting operation is carried out by connecting the terminal wires and 22 across a resistance measuring meter, not shown, which will continuously measure the resistance value of the resistance wire 24. A rotating grinding wheel, designated as 40 in Figure 2, is then brought against the outer surface of the turns of the minor portion 24b of the resistance wire 24. The grinding wheel 40 is maintained against the turns of the minor portion 24b of the resistance wire 24 to grind away the outer surface of the turns of the minor portion 24b until the resistance measuring meter indicates that the final desired resistance value has been reached. During this grinding operation, it is preferable to rotate the bobbin 12 so'that the turns of the minor portion 24b of'the resistance wire" 24 will beground away uniformly around the entire circumference of the turns.

After the resistance value of the resistance wire 24 has been adjusted to the final desired resistance value, the final step in the method of the present invention is to complete the encapsulation of the bobbin 12 and the resistance wire 24. As shown in Figure 3, this is accomplished by providing a protective jacket or casing 42 of the same material as the protective jacket 3% around the portion of the bobbin 12 which contains the shallow groove 18 to enclose the minor portion 24b of the resistance wire 24. The protective jacket 42 is bonded to and forms a continuation of the protective jacket 38. The protective jacket 42 also extends across the end of the bobbin 12 and sealingly engages the portion of the terminal wire 22 adjacent the end of the bobbin 12. The protective jacket 42 is molded or cast around the bobbin 12 in the same manner as the protective jacket 3%. Thus, the protective jackets 38 and 42 completely enclose the bobbin 12 and the resistance wire 24. The application of the protective jacket 42 will cause a mold change in the resistance value of the resistance Wire 24. However, since the protective jacket 42 surrounds only the minor portion 24b of the resistance wire 24, the mold change caused by the application of the protective jacket 42 will be only a small percentage of the resistance value of the minor portion 24b of the resistance wire 24. Since the resistance value of the minor portion 24b of the resistance Wire 24 is only a very small portion of the total resistance value of the resistance wire 24, the mold change caused by the application of the protective jacket 42 will be negligible as compared to the total resistace value of the resistance wire 24. Thus, by the method of the present invention for making the encapsulated wire wound resistor It the major variations in the resistance value of the resistor 10 caused by the skili of the operator in winding the resistance Wire on the bobbin 12, and the mold changes when the protective jacket 33 is appliedaround the major portion 24a of the resistance wire 24 are all compensated for by the adjusting operation of the method of the present invention so that the final resistance value of the resistor 10 will be more precisely the desired resistance value than can be achieved by the methods heretofore used. Also, in the mass production of the encapsulated wire wound resistor 10 of the present invention, variations in the resistance values from resistor unit to resistor unit are compensated for by the adjusting operation of the method of the present invention so that the completed resistors will all have substantially the same final resistance value.

Although the method of the present invention has been described with regard to the making of an encapsulated wire wound resistor, the method of the present invention is also applicable to making precision wire wound resistors which are not encapsulated. For making non-encapsulated wire Wound resistors by the method of the present invention, the resistance wire 24 is wound on the bobbin 12 in the same manner as previously described to provide the multi-layer major portion 24a and the single layer minor portion 24b, and the ends 26 and 28 of the wire 24 aresecured to the terminal wires 20 and 22 respectively. The amount of the resistance Wire 24 wound on the bobbin 12 is such that the resistance value of the resistance wire 24 is slightly less than the desired final resistance value. The resistance value of the resistor is then adjusted by the adjusting operation previously described in which the outer surface of the turns of the minor portion 24b of the resistance wire 24 are ground away to increase the resistance value of the resistance wire 24 to the final desired resistance value. In the making of a non-encapsulated wire wound resistor by the method of the present invention, any variations in the resistance valueof the resistor caused by the skill ofthe operator in winding the resistance wire 24 on the bobbin 1-2, and'the securing of the ends 26- and 28- of theresistance wire 24 to the terminal wires 20 and 22 are compensated for by the adjusting operation of the present invention so that the final resistance value of the resistor is substantially precisely the desired resistance value.

Although the method of the present invention is preferably applicable to making wire wound resistors in which the major portion of the resistance wire is wound in a plurality of substantially concentric layers, the method of the present invention can also be used for making wire wound resistors in which the resistance wire is wound entirely as a single layer along a bobbin. For making such a wire wound resistor, a bobbin having a cylindrical outer surface is used, and the resistance wire is helically wound around and along the bobbin in a single layer. For a non-encapsulated resistor, the resistance value of the resistance wire is then adjusted to the final desired resistance value by grinding away the outer surface of some of the turns of the resistance wire. For an encapsulated resistor, a protective jacket is applied around a major portion of the resistance wire leaving only a few turns of the resistance wire exposed. The resistance value of the resistance wire is then adjusted by grinding away the outer surface of the exposed turns of the resistance wire until the final desired resistance value is obtained.

Although in the resistor of the present invention shown in Figure 1, the bobbin 12 is provided with only one deep groove 16 for containing the major portion 24a of the resistance wire 24, the bobbin 12 may be provided with two or more deep grooves 16 for containing the major portion of the resistance wire 24 according to the amount of resistance wire to be wound on the bobbin 12. Thus, for a resistor 10 having a high resistance value, two or more of the deep grooves 16 may be necessary for containing the larger amount of the resistance wire 24 necessary to provide the high resistance value. Also, in making multi-turn Wire wound resistors, it is often desirable to wind some of the turns in a direction opposite to the other turns in order to balance out any inductance efiects in the resistance wire. Thus, by using a bobbin 12 which has two or more of the deep grooves 16, the resistance wire 24 wound in one of the grooves 16 can be wound in the direction opposite the resistance wire wound in the other groove 16 for this purpose. When making an encapsulated wire wound resistor using a bobbin 12 having two or more of the deep groove 16 for containing the major portion of the resistance wire 24, the protective jacket 38 is applied around all of the deep grooves 16 so that the mold change caused by the application of the protective jacket 38 will be applied to the total major portion of the resistance wire 24.

Although the resistor 10 of the present invention shown in Figure 1 has terminal wires extending longitudinally from the ends of the bobbin 12, the resistor 10 can be provided with any other type of terminals which are secured to the ends of the bobbin 12, and which project either longitudinally or radially from the bobbin 12. Also, the ends 26 and 28 of the resistance wire 24 do not have to extend through angled holes in the bobbin 12 to the terminals, but can be connected to the terminals by any other well known method according to the type of terminals used.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. A method of making an encapsulated wire wound electrical resistor comprising the steps of winding an electrical resistance wire around a bobbin having terminals secured to the ends of the bobbin, said resistance wire being wound around said bobbin with at least a portion of said resistance wire being wound as a single layer, connecting the ends of said resistance wire to said terminals, casting a protective jacket of an electrical insulating material directly around the major portion of said resistance wire leaving at least a portion of the turns of said resistance wire which are wound as a single layer exposed, then adjusting the resistance value of said resistance wire to the final desired resistance value by grinding away the outer surface of the exposed turns of the resistance wire, and then casting a protective jacket of the electrical insulating material directly around the exposed turns of the resistance wire to completely enclose the resistance wire.

2. The method in accordance with claim 1 in which an amount of the resistance wire is wound on the bobbin so that the resistance value of the resistance wire just prior to the adjustment of the resistance value of the resistance wire is slightly less than the final desired resistance value.

3. A method of making an encapsulated wire wound electrical resistor comprising the steps of winding an electrical resistance wire around a bobbin having terminals secured to the ends of the bobbin, the major portion of said resistance wire being wound around a portion of said bobbin in a plurality of substantially concentric layers, and the remaining minor portion of said wire being wound around another portion of said bobbin in a single layer, connecting the ends of said resistance wire to said terminals, casting a protective jacket of an electrical insulating material directly around the major portion of said resistance wire leaving the minor single layer portion of said resistance wire exposed, then adjusting the resistance value of said resistance wire to the final desired resistance value by grinding away the outer surface of the exposed turns of the resistance wire, and then casting a protective jacket of the electrical insulating material directly around the minor portion of the resistance wire to completely enclose the resistance wire.

References Cited in the file of this patent UNITED STATES PATENTS 1,985,691 Pugh Dec. 25, 1934 2,500,605 DeLange et al. Mar. 14, 1950 2,547,405 Mitchell et a1. Apr. 3, 1951 2,792,620 Kohring May 21, 1957 2,868,934 Aske Jan. 13, 1959 

